Electrode substrate made of carbon fibers and method of producing the electrode substrate

ABSTRACT

A porous electrode substrate has a form of a tape material and contains a structure made of carbon fibers and a carbon matrix. A specific surface area, porosity, and pore distribution are determined by the carbon matrix. The carbon matrix contains carbon particles including activated carbon with a high specific surface area and a carbonized or graphitized residue of a carbonizable or graphitizable binder.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application, under 35 U.S.C. §120, of copendinginternational application No. PCT/EP2014/068148, filed Aug. 27, 2014,which designated the United States; this application also claims thepriority, under 35 U.S.C. §119, of German patent application No. DE 102013 217 882.4, filed Sep. 6, 2013; the prior applications are herewithincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an electrode substrate made of carbonfibers—especially, for redox flow batteries, regenerative fuel cells,polymer electrolyte fuel cells, metal-air batteries, lithium sulfurbatteries, or zinc-bromine batteries—as well as to methods for theirproduction.

Redox flow batteries and regenerative (or reversible) fuel cells areelectrochemical energy storage devices that feature aqueous or organicsolutions of metal ions and transition metal ions and/or respectivecomplexes such as, for example, from iron, cerium, chrome, zinc, titan,vanadium, or redox systems on the basis of solutions of halogens (forexample, bromide and/or chloride) or polysulfides as active mass forcharge storage. These are stored in external tanks and pumped through anelectrochemical reactor containing two half-cells divided by anion-conducting membrane or a micro-porous separator during batteryoperation.

Porous flow electrodes perfused by the solutions of the active materialsare arranged in the half-cells.

During charging and/or discharging, the respective redox pairs in thehalf-cells are oxidized and/or reduced on porous flow electrodes (carbonfelt or carbon foam, metal foams). Due to the external storage of theactive masses and the energy conversion in a reactor (cell), independentdimensioning of energy and performance of the battery system becomespossible.

Similar to redox flow batteries, fuel cells convert externally suppliedfuel, such as hydrogen or low alcohols/ethers, and an oxidationagent—usually oxygen or air—to electricity, reaction products, andexcess heat by electrochemical reactions.

For fuel cells and redox flow batteries, several individual cells, dueto the low cell potential, are usually combined into a cell stack inorder to increase voltage and performance output. There exist,furthermore, hybrid forms, such as, for example, a vanadium-air fuelcell in which the negative electrode consists of a vanadium-salinesolution in a porous flow electrode and the positive electrode (cathode)is configured analogously to a classical fuel cell cathode.

Metal-air batteries combine an anode made of an electropositive metal(lithium, zinc) with the redox partner, oxygen, electrochemicallyreacting on a cathode, in the form of a carbon-based or metal-based gasdiffusion electrode, to give water or hydroxide ions. Gas diffusionelectrodes are usually rendered water-repellent by impregnation withfluorine-polymers and to provide a three-phase boundary (solid electronconductor/liquid phase/gas phase) on which the electrochemical reactionmay take place.

Metal sulfur batteries are secondary batteries combining lithium orsodium as the negative electrode, with the redox systemsulfur/polysulfide as the positive electrode. Since the active materialsulfur has little or no electrical conductivity, it is introduced into aporous, powdery carbon matrix (lithium-sulfur battery) and/or a carbonfelt (sodium-sulfur battery).

With any electrode material, it is desirable that the electrodesubstrate may be processed as continuous roll material in order toachieve high production volumes. This allows the application ofcost-effective procedures on an industrial scale, e.g., for depositingpotentially required catalyst layers on the substrate, and forsubsequent production steps such as, for instance, lamination on acurrent arrester. Furthermore, an electrode substrate as a continuousroll offers higher homogeneity and uniformity of the product, comparedwith electrode substrate produced in batches.

In addition to the requirements discussed above, the electrode materialmust be inert and corrosion resistant with regard to the electrolytematerials, fuels, oxidizing agents, and reaction-products and/orby-products. Moreover, the flexibility of the electrode material must besufficient to allow processing from coil to coil as roll material.

The disadvantage of the usual electrode materials for redox flowbatteries is their thickness of 2 to 5 mm, which leads to a relativelyhigh electrical resistance, as well as to a significant hydrodynamicresistance when perfused. Furthermore, those materials cannot usually besufficiently wetted for the electrolyte solutions and must be partiallyoxidized by thermal treatment before use.

A new approach for redox flow batteries is to configure the battery witha current arrester made of a graphite plate, with current channelscombined with a thin electrode on a carbon fiber basis (D. S. Aaron etal., Journal of Power Sources, volume 206, 2012, 450-453). This conceptallows a considerably higher power density due to the reduced ohmicresistors, compared to the traditional design.

For metal-air batteries, gas diffusion electrodes are usually producedby coating a carrier structure (e.g., PTFE or metallic gauze) withcarbon particles.

Positive electrodes of lithium sulfur accumulators are usuallyconfigured as paste electrodes, for which a suspension of carbon andsulfur particles is coated onto a metal-based arrester foil.

SUMMARY OF THE INVENTION

It is, therefore, the purpose of the invention to find a cost-efficientand flexible method of production for a thin, fiber-based, and connectedelectrode structure, which may be used for redox flow batteries,regenerative fuel cells, metal air, metal sulfur batteries, or fuelcells, depending upon the embodiment.

The method is intended to optimize the electrode material duringproduction by applying catalytically active substances (redox flowbattery, metal air battery), inhibitors (redox flow battery), orfunctional carbon-based filler materials (redox flow battery, lithiumsulfur battery), depending upon the respective application.

In addition, materials should be such as may be flexibly processedfurther, e.g., by lamination with a current arrester as a plate or film.

Catalytically active substances (e.g., Co, Ir, Pt, Bi, Mn, Te, In, Pd)accelerate the electrochemical reactions on the electrode (e.g., redoxreactions of transition metal ions, metal ions, and the oxygen reductionof metal air batteries) and thus increase the battery performance.

A higher specific surface area of the electrode, caused by the fillingmaterial matrix, and/or heteroatom doping (oxygen, nitrogen) of thecarbon structure has similarly positive effects upon the electrochemicalkinetics.

Inhibitors (particles, salts, or oxides of Pb, Bi, Sn, Cd, Tl, In, Sb,Au) increase the load efficiency of redox flow batteries, since theyobstruct parasitic reactions such as the formation of hydrogen in theelectrolyte (see European patent EP 0312875 A1).

For optimum efficiency of a lithium sulfur battery, a certain porestructure and a defined pore space are required (Hagen et al., Journalof Power Sources, volume 224, 2013, 260-268). This may be custom-madeduring the process by selection of the filler material matrix (type,quantity).

The subject-matter of the invention is a porous electrode substrate inline form (roll material), containing a structure of carbon fibers and acarbon matrix which is generated by, for example, impregnating the linewith a preferably water-based or alcoholic dispersion of carbonparticles, binder substances, and doping agents (e.g., metal particlesor metal oxide particles, pore-forming agents) and subsequent hardeningand/or carbonization.

The specific surface area, porosity, and pore distribution aredetermined by the carbon matrix according to the invention, preferablycontaining the filling material type, the filling material content,and/or the temperature for thermal treatment.

The mass ratio of binder substances to carbon particles is preferablybetween 1:10 and 10:1.

Pore-forming agents are preferably selected from the group ammoniumhydrogen carbonate, ammonium carbonate, lithium carbonate, ammoniumoxalate, ammonium acetate, oxalic acid, azodicarbonamide, azoisobutylnitrile, benzoyl peroxide, cellulose powder, micro-crystallinecellulose, sucrose, and starch flour.

Preferably, at least a portion of the interstices in the structure ofcarbon fibers or carbon precursor fibers and the carbon matrix is filledwith an activated carbon with high specific surface area, as well aswith a carbonized or graphitized residue of a carbonizable orgraphitizable binder.

BET measurement is a term for a procedure for analyzing the size ofsurfaces, especially of porous solid bodies, by gas adsorption. It is amethod from surface chemistry used to calculate the mass-relatedspecific surface area from experimental data.

It is preferred that the mass relation between the carbonized orgraphitized residue and the carbon particles, as well as the activatedcarbon with high specific surface area, should be between 1:10 and 10:1and the carbonized or graphitized residue, together with the carbonparticles and the activated carbon with high specific surface area,constitute a mass proportion between 25 and 75% on the electrode, thatthe substrate BET be 5 to 250 m²/g preferably, in particular, 1 to 100m²/g that the porous electrode substrate be between 0.1 and 0.4 mmthick, and that the electrical resistance in z-direction be <25 mOhm/cm²preferably, <10 mOhm/cm².

It is particularly preferable to select the structure of carbon fibersfrom the group non-crimp fabric, paper, woven fabric and nonwoven. Thethickness of the woven fabric or nonwoven is preferably between 0.1 and0.6 mm.

It is particularly preferable that the carbon particles consist ofacetylene soot, oil soot, gas soot, graphitized soot, ground carbonfibers, carbon nanotubes (CNT's), carbon nano-fibers, carbon aero gels,meso-porous carbon, fine-grain graphite, glassy carbon powder, expandedgraphite, ground expanded graphite, graphite oxide, flake graphite,activated carbon, graphene, graphene oxide, N-doped CNT's, boron-dopedCNT's, fullerenes, petcoke, acetylene coke, anthracite coke, carbonizedmeso-phase pitches, and/or doped diamond.

The carbonizable or graphitizable binder preferably consists of hardcoal tar pitches, phenol resins, benzoxazine resins, epoxide resins,furan resins, furfuryl alcohols, vinyl ester resins and particularlypreferred of materials that have heteroatoms in the carbon structure andmay generate heteroatom-doped carbon during carbonization, such asmelamine-formaldehyde resins (MF), urea-formaldehyde resins (UF),resorcinol formaldehyde (RF) resins, acrylonitrile butadiene rubber,cyanate-ester resins, bismaleimide resins, polyurethane resins, and/orpolyacryle nitrile.

It is particularly preferred that the carbon proportion as carbonfibers, carbonized, or graphitized residue, as well as the carbonparticles and the activated carbon, be at least 95% by weight and theheteroatom proportion at least 1% by weight.

Porosity is preferably between 15 and 97% by weight, expressed by therelation of the open volume to the sum of open volume, volume of carbonfibers, and the volume formed by all solid materials, containingcarbonized or graphitized residue, as well as the carbon particles andactivated carbon.

Porous carbon may preferably be generated with poly-vinylidene fluoride(PVDF).

In addition, the porous electrode substrate was, preferably, impregnatedwith one or several impregnating agents and/or doped with one or severaldoping agents.

The electrode substrate is, preferably, also additionally coated with alayer consisting of completely or partially fluorinated polymer, as wellas conductive particles.

Another subject matter of the invention is a method for producing aporous electrode substrate in line form (roll material), wherein aprecursor structure is carbonized and the resulting structure of carbonfibers is impregnated and dried and/or hardened with a dispersioncomprising carbon particles, a carbonizable binder and activated carbon,and carbonized in an inert gas atmosphere, preferably with an inert gassuch as, for example, nitrogen or argon, in a continuous furnace at800-3,000° C.—preferably at 900-2,000° C.

The precursor structure consists, preferably, of carbon fibers, fibersfrom the group of polyacrile nitrile, oxidized polyacryl nitrile(PANOX), Novoloid (phenole resin fibers), cellulose, cellulose acetate,lignine, polyaramide, polyimide, polyoxodiazole, polyvinyl alcohol,polyamide, or pitch fibers.

It is particularly preferred that the carbon fibers used are short cutfibers, stable fibers, or continuous filaments.

The carbon fiber proportion in the structure is preferably 10 to90%—most preferably, 20 to 80%.

Impregnation takes place, preferably, with a water-based or alcoholicdispersion.

Preferably, the impregnating agent contains a hydrophobe polymer, andthe share in the electrode substrate is between 2 and 40% by weight.

In another embodiment, the carbonized substrate is, preferably,impregnated with fluoride-dispersions (for example, PTFE, PVDF, ETFE,and/or PFA) or poly-siloxane dispersion.

It is furthermore preferred that impregnation take place with H₂inhibitors, among them nanoparticles of Au, Bi, Pb, Sn, Sb, In(III)oxide, In(III) salts, Bi(III) oxide, Bi(III) salts, Bi(OH)₃, In(OH)₃,antimony(III) oxide, antimony (III) salts, lead and lead salts, leadacetate, lead sulfate, tin(II) oxide, and Mo(II) salts.

Impregnation may be effected, preferably, by evaporation deposition orplasma deposition (PVD) and/or by galvanic or current less deposition ofAu, Sn, Pb, Bi, and/or Sb.

Further preferred is doping with metals and/or metal oxides, among themmanganese (III) salts, manganocene, cobaltocene, manganese(III, IV)oxides, nano-scale bismuth oxide(Bi₂O₃), ruthenium salts/ruthenium(II)oxide, RuO₂, iridium(III) salts, iridium oxide, metal particles andmetal nanoparticles—preferably, in particular, Au, Sn, Pb, Bi, Ru, Ag,Rh, In, Sb, Ir, and/or Pd, polyoxometalate (POM), co-porphyrine, iridiumoxide, Raney nickel, boron trioxide, boron acid, borax, boron acidester, titanium diboride, sol-gel dispersions on the basis of alkoxidesfrom the group Mn, Bi, Co, Ti and B, molybdenum disulphide, and Mo(II)salts.

Doping takes place, preferably, by evaporation deposition or plasmadeposition (PVD) or galvanic and/or current less deposition of metalsand/or metal oxides, such as Co, IR, manganese oxide and/or Au, Bi, Sn,Pb, In, Sb.

The impregnating dispersion preferably contains one or more dispersionagents selected from lignine sulfonates, naphtaline sulfonatecondensates, polyalkylphenyl ether, polyethylene oxide polypropyleneoxide copolymers, polyacrylate and polyvinyl alcohols, as well asparticularly preferred dispersion agents containing N such as polyvinylpyrrolidone, polyethylenimine, polyaminobenzol sulfonic acid, polybenzylviologenes, or polydiallyldimethyl ammonium chloride.

Hardening agents used are, preferably, hexamethylene tetramine,ethylenediamine, glutardialdehyde, paraformaldehyde, or terephtaldehyde.

In a preferred embodiment, several impregnated layers of the electrodesubstrate are placed one upon another and hardened at >120° C. Followingthat, carbonization at 900 to 1,800° C. is particularly preferred.Optionally, surface oxidation at, for example, 450° C. for a maximumperiod of 15 minutes may occur in air and/or under air supply.

In another preferred embodiment, the surface oxidation is effectedchemically, for example with nitric acid, perchloric acid, periodate,permanganate, Cer(IV) sulfate, or hydrogen peroxide.

The substrate is, preferably, oxidated thermally after the carbonizationstep—for example, with air and/or oxygen and, for example, at atemperature of >250° C. or in a wet-chemical process.

It is particularly preferable that the porous electrode substrate beused in redox flow batteries and/or lithium sulfur batteries and/orsodium sulfur batteries and/or zinc bromine batteries and/or zinc airbatteries and/or vanadium air batteries and/or vanadium air fuel cellsand/or polymer electrolyte fuel cells and/or microbial fuel cells and/orH₂/Cl₂ fuel cells and/or H₂/Br₂ fuel cells and/or PEM electrolyzers.

For redox flow batteries, the use of a laminate with a current collector(e.g., graphite plates or graphite compound bi-polar plates) or arresterfoil (e.g., Ti, Ni, or graphite film) is preferable. Further preferredare simple and/or multiple material layers or the grouting beforecarbonization for multi-layer embodiments.

For lithium sulfur batteries and sodium sulfur batteries, infiltrationpreferably occurs with sulfur, thiosulfate, xanthogenates, and/orpolysulfides.

For zinc air batteries, vanadium air batteries, and fuel cells,hydrophobization preferably takes place with fluoride polymers orsiloxane polymers.

Since the specific surface area, porosity, and pore distribution may bedetermined by the carbon matrix (filler matrix) and/or the temperatureduring thermal treatment, an adaptation to the various operational areasis, surprisingly, possible.

For example, for redox flow batteries, a high BET, a higher catalyticactivity of heteroatoms in the C grid, and doping with inorganic oxidesand/or metal particles is desired—for lithium sulfur batteries, amid-level BET.

DETAILED DESCRIPTION OF THE INVENTION

The following examples explain the invention.

EXAMPLE 1

A line of carbon fiber raw paper (square measures 20 g/m²), produced inwet fluid processing with short cut carbon fibers (3-12 mm), isimpregnated by a foulard in an aqueous dispersion consisting of 50 kgwater, 0.75 kg polyvinyl pyrrolidone, 6.75 kg acetylene soot, 0.75 kgactivated carbon (BET>1,000 m²/g), 0.75 kg 2-aminopropanole, 0.1 kgammonium hydrogen carbonate, and 18.75 kg resorcinol formaldehyde resindispersion and dried and/or hardened in a continuous furnace.Carbonization subsequently takes place under inert gas atmosphere in acontinuous furnace at 1,400° C.

EXAMPLE 2

A roll of carbon fiber fleece (40 g/m²), produced by carbonizing awater-jet hardened fleece on the basis of polyacryl nitrile or oxidizedpolyacryl nitrile staple fibers (20 to 80 mm), is impregnated on afoulard in an aqueous dispersion consisting of 56 kg water, 0.95 kgpolyvinyl alcohol, 7.5 kg acetylene soot (BET surface 60 m²/g), and20.65 kg melamine formaldehyde resin (40%) and dried and/or hardened ina continuous furnace. Carbonization subsequently takes place under inertgas atmosphere in a continuous furnace at 1,400° C.

The following table shows the material parameters for examples 1 and 2,with a reference sample for comparison. A 2-point measurement at a loadof 100 N/cm² was conducted to measure the resistance.

Resistance Element analysis Thickness Porosity (mOhm/ C BET (μm) (%)cm²) O N (m²/g) Reference 370 87.8 4.2 98.5 0.3 0.8 GDL 0.1 10AA Example210 91.2 6.2 97.8 0.9 70 1 0.6 Example 380 88.7 9.8 97.5 1.4 34 2 0.1

In order to assess electrochemical activity, cyclic voltammetrymeasurements of untreated electrode materials were conducted in 1 mMFe(CN)₆ ^(3−/4−) in 0.1 M potassium chloride solution.

An ideally reversible redox pair results in a separation of 60 mVbetween the oxidation (E_(p) ^(ox)) and reduction peak (E_(p) ^(red))(A. J. Bard, L. M. Faulkner (eds.), Electrochemical Methods:Fundamentals and Applications, Wiley, 2001). The considerably smallerpeak separations for the materials in examples 1 and 2, compared to thereference material, confirm the significantly improved electrochemicalkinetics of the materials from embodiment examples 1 and 2.

E_(p) ^(ox) - E_(p) ^(red) Reference GDL 10AA 322 mV Example 1  70 mVExample 2 100 mV

1. A porous electrode substrate formed as a tape material, comprising: astructure of carbon fibers; and a carbon matrix, a specific surfacearea, a porosity, and a pore distribution being determined by saidcarbon matrix.
 2. The porous electrode substrate according to claim 1,wherein said carbon matrix contains carbon particles including activatedcarbon with a high specific surface area and a carbonized or graphitizedresidue of a carbonizable or graphitizable binder and that at least apart of interstices in said structure of carbon fibers and said carbonmatrix is filled with said activated carbon and said carbonized orgraphitized residue of said carbonizable or graphitizable binder.
 3. Theporous electrode substrate according to claim 2, wherein: a mass ratiobetween said carbonized or graphitized residue and said carbonparticles, including said activated carbon with the high specificsurface area, should be between 1:10 and 10:1; and said carbonized orgraphitized residue, together with said carbon particles, constitute amass proportion between 25 and 75% of the porous electrode substrate, asubstrate BET is 5 to 250 m²/g, the porous electrode substrate has athickness between 0.1 and 0.4 mm and an electrical resistance in az-direction is below 25 mOhm/cm².
 4. The porous electrode substrateaccording to claim 1, wherein said structure of carbon fibers isselected from the group consisting of non-crimp fabrics, paper, wovenfabrics and nonwovens.
 5. The porous electrode substrate according toclaim 2, wherein said carbon particles contain at least one of acetyleneblack, furnace black, gas black, graphitized carbon black, milled carbonfibers, carbon nanotubes (CNT's), carbon nano-fibers, carbon aerogels,meso-porous carbon, fine-grain graphite, glassy carbon powder, expandedgraphite, ground expanded graphite, graphite oxide, flake graphite,activated carbon, graphene, graphene oxide, N-doped CNT's, boron-dopedCNT's, fullerenes, petcoke, acetylene coke, anthracite coke, carbonizedmeso-phase pitches, or doped diamond.
 6. The porous electrode substrateaccording to claim 2, wherein said carbonizable or graphitizable bindercontains at least one of coal tar pitches, phenolic resins, benzoxazineresins, epoxide resins, furane resins, furfuryl alcohols, vinyl esterresins, melamine-formaldehyde resins (MF), urea-formaldehyde resins(UF), resorcinol formaldehyde (RF) resins, acrylonitrile butadienerubber, cyanate-ester resins, bismaleimide resins, polyurethane resins,or polyacrylonitrile.
 7. The porous electrode substrate according toclaim 2, wherein a carbon proportion in a form of said carbon fibers,said carbonized or graphitized residue and said carbon particlesincluding said activated carbon, is at least 95% by weight and aheteroatom proportion is at least 1% by weight.
 8. The porous electrodesubstrate according to claim 2, wherein the porosity is between 15 and97% by weight, expressed as a proportion of an open volume to a sum ofopen volume, volume of carbon fibers, and a volume formed by all solidmaterials, containing said carbonized or graphitized residue and saidcarbon particles including said activated carbon.
 9. The porouselectrode substrate according to claim 1, wherein the porous electrodesubstrate is at least one of impregnated with at least one impregnationagent or doped with at least one doping agent.
 10. A method forproducing a porous electrode substrate as a line material, whichcomprises the steps of: carbonizing a precursor fiber structureresulting a structure of carbon fibers; and performing at least one ofimpregnating, drying or hardening the structure of carbon fibers with adispersion containing carbon particles including activated carbon and acarbonizable binder resulting in an impregnated structure of carbonfibers; carbonizing in a continuous furnace at 800-3,000° C. in an inertgas atmosphere the impregnated structure of carbon fibers resulting in acarbonized structure.
 11. The method according to claim 10, wherein theprecursor fiber structure has fibers selected from the group consistingof polyacryl nitrile fibers, oxidized polyacryl nitrile fibers (PANOX),Novoloid (phenol resin fibers), cellulose fibers, cellulose acetatefibers, lignine fibers, polyaramide fibers, polyimide fibers,polyoxodiazole fibers, polyvinyl alcohol fibers, polyamide fibers, andpitch fibers.
 12. The method according to claim 10, wherein the carbonfibers are short cut fibers, staple fibers or continuous filaments. 13.The method according to claim 10, which further comprises setting acarbon fiber proportion in the precursor fiber structure to be 10 to90%.
 14. The method according to claim 10, wherein the dispersioncontains at least one dispersion agent selected from the groupconsisting of lignine sulfonates, naphtaline sulfonate condensates,polyalkylphenyl ether, polyethylene oxide polypropylene oxidecopolymers, polyacrylate and polyvinyl alcohols, polyvinyl pyrrolidone,polyethylenimine, polyaminobenzol sulfonic acid, polybenzyl viologenesand polydiallyldimethyl ammonium chloride.
 15. The method according toclaim 10, wherein the carbonized substrate is additionally impregnatedwith at least one impregnating agent.
 16. The method according to claim10, which further comprises doping the structure of carbon fibers withat least one doping agent.
 17. The method according to claim 15, whereinthe impregnating agent contains a water-repellent polymer, and aproportion of the impregnating agent in the porous electrode substrateis between 2 and 40% by weight.
 18. The method according to claim 16,wherein the dispersion additionally contains the at least one dopingagent, the at least one doping agent containing at least one of H₂inhibitors, metals, metal salts, or metal oxides.
 19. The methodaccording to claim 11, wherein the carbonized structure is thermally orwet-chemically oxidized after the carbonizing step.
 20. An apparatusselected from the group consisting of redox flow batteries, lithiumsulfur batteries, sodium sulfur batteries Z, zinc bromine batteries,zinc air batteries, vanadium air batteries, fuel cells, microbial fuelcells, H₂/Cl₂ fuel cells, H₂/Br₂ fuel cells, and PEM electrolyzers, theapparatus comprising: a porous electrode substrate formed as a tapematerial, said porous electrode containing a structure of carbon fibersand a carbon matrix, a specific surface area, a porosity, and a poredistribution being determined by said carbon matrix.